In recent years, as interest in the efficient use of energy has increased, there have been efforts to increase the efficiency of motors which are used in electrical systems, including large-capacity electric generators and environmentally friendly vehicles such as hybrid electric vehicles (HEVs) or electric vehicles (EVs). For example, there has been an effort to modulate the frequency of BLDC motors to obtain a higher rotating speed than that of general motors.
Particularly, in the case of motors which are used in the driving unit of hybrid vehicles or electric vehicles, it is required to obtain a large output with a limited size, and a rotating speed of 10,000 rpm or more is required. In this case, a centrifugal force which is applied to the rotator of the motor is proportional to the square of the rotating speed, and thus exceeds the yield strength of general electrical steel sheets during high-speed rotation and threatens the stability and durability of the motors. Thus, the rotator of high-speed rotating devices requires a high-strength material.
In addition, in the case of materials that are used for the rotator of motors, an eddy current loss caused by high frequency is required to be reduced in addition to increasing the strength. When a high-strength carbon steel or integral rotator is made in order to increase the strength, the eddy current loss of the rotator increases to reduce the overall efficiency of the motor.
Thus, there has been a need for studies on the electrical steel sheet manufacturing technology capable of satisfying both high-strength properties and low core loss properties. For example, a technology of increasing strength by forming structures other than ferrite in steel, a technology of increasing alloying elements such as Nb, V and C to steel, and a technology of satisfying both core loss properties and strength properties by controlling the grain size to 20 μm or more before cold rolling or additional processing have been proposed.
However, the technology of forming structures other than ferrite has shortcomings in that, because nonmagnetic abnormal structures such as pearlite, martensite or austenite are present in the steel, the core loss and magnetic flux density of the steel are rapidly deteriorated, and the efficiency of a motor employing the steel decreases rapidly. In addition, the technology of adding alloying elements such as Nb, V or Cu has shortcomings in that the magnetic properties of the steel are rapidly deteriorated, and limitations occur in some applications. Further, the effect of the technology of controlling the size of cold-rolled structures to 20 μm or more appears in processes, which are performed on conventional electrical steel sheets, and intermediate products. The results of experiments conducted by the present inventors showed that the effect of the technology was insignificant on high-strength electrical steel sheets having a large amount of non-recrystallized structures and that it is difficult to improve the magnetic properties of the steel, compared to those of a material having a grain size of less than 20 μm.